Characterisation of Solid Oxide Fuel Cells and Electrodes Using EIS Mogens Mogensen Materials Research Department, Risø National Laboratory DK-4000 Roskilde, Denmark Presentation at the International Workshop ”Impedance Spectroscopy for Characterisation of Materials and Structures Warsaw, 24 – 28 September, 2003

Introduction Electrochemical impedance spectroscopy, EIS, is in many respects a strong tool for SOFC characterisation, but it certainly has its limitations Difficult to interpret one or few spectra unless other information is available Very little physical and chemical information available in one spectrum

How to model or fit EI spectra? 1) Equivalent circuits? or 2) Physical-chemical models? The answer is yes! Naturally, we should end up with 2), but 1) may be a necessary intermediate step.

SOFC EIS examples In the following important features of the efforts of revealing the SOFC electrode mechanisms here at Risø is given; mainly the Ni-YSZ-H 2 /H 2 O is used as example. Details are found in the ph.d. theses of Søren Primdahl Karin Vels Jensen (now Hansen) Mette Juhl Jørgensen Most of it has been and some is being published in the open literature

Examples of H 2 /3%H 2 O/Ni-YSZ at 1000°C. a) 50/50 vol % Ni/YSZ fine powders, b) Risø ”standard”. 1Hz

Q = Y 0 (j  ) n, Y 0 and n are constants,  = 2  f For a given electrode n 1, n 2, n 3 …. should be constant or develop smoothly with varied parameter!!!

Error plots for the two previous H 2 /3%H 2 O/Ni-YSZ EIS using this equivalent circuit, (n 1, n 2, n 3 ) = (0.8, 0.75, 1).

The Risø three- electrode set-up with a separate reference gas for the reference and counter electrodes

Impedance spectrum of Ni -YSZ TPB IT proces Gas diffusion Gas conversion

Test set-up for electrodes in a symmetric cell with typical area of 0.25 cm 2

Two Risø three- electrode pellets in one atmospere placed working to working electrode to aviod change in local gas composition to the larges possible extent

Gas conversion impedance

The TPB IT (ion transfer) process? - Literature on H 2 /Ni/YSZ - Extreme disagreement Activation energies from eV! Dependencies on partial pressures of water and hydrogen vary a lot. For hydrogen both negative and positive dependencies have been found Do people study different systems even though they believe that they are identical?

XPS of YSZ surface. After Badwell and Drennan, Y Si Ti Na 50 h

The TPB CT process The old ”pure” ideas and actual practical reaction limitations

EIS of air/LSM-YSZ/YSZ electrodes. 1000°C, 0.4 cm 2. 1 Hz 100 Hz 10 kHz 10 Hz

May consist of at least five arcs * : Arc A and B is charge transfer Arc C is oxygen reduction / oxidation Arc D is oxygen diffusion Arc E is some kind of activation i.e. depassivation EIS of O 2 /O 2- / LSM+YSZ/YSZ

EIS on a 16 cm 2 AS-SOFC at 850 °C

Conclusion You do not know in details which processes you are studying as the starting point in real research Therefore, forget about first-principle- modelling until you have done the many experiments which show the nature of the operating processes